This paper develops an intake two-pipeline model on a 125 cc single engine including the effects of engine torque (T), fuel consumption (FC) and both CO and HC emission. Those four performances in prescribed speed range were constructed by the response surface method. The formal design of experiment has been utilized to examine the significances and characteristics of interrelated parameters. A proposed engineering design optimization simultaneously treats four performances with weighting strategy so that the optimum two-pipeline diameters, lengths, and the appropriate timing of switching low-speed pipe to high-speed pipe can be obtained. The presented integration process of experimental design optimization offers an applicable process and technique for small engine and can apply to other mechanical product development.